The vibration-generating mechanisms inside an engine are highly non-linear (combustion, valve operation, hydraulic bearing behavior, etc.). However, the engine structure, under the influence of these vibration-generating mechanisms, responds in a highly linear way.

For the development and optimization of the engine structure for noise and vibration it is beneficial to use fast and ‘simple’ linear models, like linear FE-models, measured modal models or measured FRF-models.

All these models allow a qualitative assessment of variants without excitation information. But, for true optimization, internal excitation spectra are needed in order to avoid that effort is spent to optimize non-critical system properties.

Unfortunately, these internal excitation spectra are difficult to measure. Direct measurement of combustion pressure is still feasible, but crank-bearing forces, piston guidance forces etc. can only be identified indirectly. Inverse identification of the main internal excitations through operational acceleration measurements, combined with laboratory tests of FRF matrices, is the only possibility.

This paper discusses inverse engine internal force identification using operational accelerations and laboratory FRF-matrices and the use of engine FRF matrices to assess modifications and variants with and without this excitation information.